One more thing to keep in mind:

From William Page's book:

"In object-level fragmentation, the object might look fine and healthy at the tablespace level, having a single extent, but some probing inside those extents can reveal a different story.

A common occurence is that a DBA sees space allocation problems in an often-modified tablespace, and rebuilds the tablespace and all its objects. Subsequently, the database performance can increase. This increase in performance is attributed to the tablespace fragmentation being eliminated, but it could very well be the eliminition of object-level fragmentation that gives you the performance boost."

Mr. Page says that in order to de-fragment the tablespace, do an export and import of the objects within that tablespace. The export/import process "not only coalesces your free space, but Export also coalesces your database objects into one extent." So if you were asked to de-frgment the tablespace because of performance issues, then you might want to consider export/import to solve two problems in one shot.